(1) Field of the Invention
This invention relates to the detection of sample fluid on an analysis slide of the type which is adapted to receive a predetermined quantity of sample fluid and to produce a response proportional to a selected analyte in the fluid. More particularly, the invention relates to a method and apparatus for determining whether a sufficient amount of fluid has been metered onto the analysis slide to produce a suitable response.
(2) State of the Prior Art
Recent developments have provided generally planar analysis slides which are used for the detection of selected analytes in biological fluids. The slides are adapted to be used in automatic high-throughput analyzers. In the processing of such slides, a predetermined amount of biological fluid, for example 10 microliters, is metered onto the slide, and a reflectance reading of the slide is taken after a suitable incubation period. The volume of fluid metered onto the slide must be controlled within certain limits to obtain acceptable test results. Thus, it is important to know if the metering apparatus is functioning within these limits.
Metering devices, for use in clinical apparatus, are adapted to expel fluid from a metering tip by means of a pressure system. One method of determining whether a metering device is functioning properly is to monitor the pressure change in the system as the fluid is being metered. It has been found, however, that such a method is not sufficiently sensitive for the precision tests being performed in most analyzers. Further, this method does not detect whether or not the fluid has been deposited in the proper area of the analysis slide. A metering device having a pressure detector of the type described is disclosed in U.S. Pat. No. 4,041,995.
Devices for measuring the moisture content of sheet material, such as paper, are well known. For example, U.S. Pat. No. 3,471,698, discloses an infrared absorption analyzer for detecting, on a reflecting surface, the presence of a thin film of a contaminant which has a defined infrared absorption band. Radiation covering selected portions of the infrared spectrum is projected onto, and reflected from, the surface to be tested. The reflected radiation is monitored to determine the amount of radiation reflected in that wavelength band in which the contaminant to be detected has a strong infrared absorption, and the amount of infrared radiation reflected in an adjacent wavelength band in which the contaminant does not show a strong absorption. A comparison of the amount of radiation reflected in the two bands will give an indication of the amount of contaminant present on the surface.
A moisture analyzer, as described above, requires complex optical and sensing devices and is not suitable for use in clinical apparatus. Further, several structurally different types of slides are required for the various analytes, and each type of slide has a reflectivity different from the other types. Thus, in certain cases, the reflectivity of a wet slide of one type may be very close to the reflectivity of a dry slide of another type. The apparatus for detecting fluid on analysis slides must be able to correctly process signals from all types of slides.